As with other herpes viruses, infection with CMV is thought to result primarily from reactivation of latent virus. Transmission of the virus can also occur from donor marrow infusion or from allogeneic red cell, leukocyte or platelet transfusions. However, in immunocompromised bone marrow transplant recipients, CMV is frequently reactivated and disease resulting from the progression of CMV infection is a major cause of infectious morbidity and mortality. CMV infection is a consequence both of the immunosuppression these patients receive and also may reflect delayed immune reconstitution in these patients following transplant. It is of particular concern to recipients of VUD (HLA-Volunteer Unrelated Donor) transplants who often receive lower doses of donor T cells in the transplant and who also require greater post-transplant immunosuppression for GVHD prophylaxis. CMV reactivation is most frequent in this group of transplant recipient.